When working on a drawing board, you use a specific scale to draw diagrams. For example, when you need to draw a plan of a building or a township, you reduce the size of the diagrams to 1/100 or 1/1000 of its actual size, that is, you use a 1:100 or 1:1000 scale. When you need to draw a small machine part, you draw it many times larger than its actual size. CADD uses the same principle to scale the drawings; however, a different approach is taken.

All CADD drawings are created on a full scale (1:1). Even if you have to draw the map of a township, you will draw it by entering the actual measurements. You can draw as big or small on the screen as you like and adjust the image using view-display functions. The magnification of the image has no relevance to the actual scale of the drawing. The actual scale of the drawing is determined when the drawings are plotted. If you drew a 36'-0" x 24'-0" rectangle and you want to fit it on a 36" x 24" paper, you need to scale it down to 1/12th, that is, you need to apply 1" = 1'-0" scale.

The dimensions you enter on-screen are called “drawing units” (or master units); the actual size that is printed on paper is measured with “plotter units”. When you enter the plot function, the plot dialog box opens to allow you to specify the ratio between drawing units and plotter units. The ratio between the plotting units and drawing units is called “scale factor”. The scale factor determines how big or small the drawing will be printed. The following table shows some of the common scales used in architectural and engineering drawings and the scale factors associated with them:

When starting a drawing, one of the first steps is to determine the plotting scale and sheet size. This is calculated the same way as it is when working on a drawing board. You choose a scale and a sheet size based on the size of the diagrams. You can calculate the maximum size diagram that can fit on a sheet by multiplying the scale factor with the sheet size.

Once the plotting scale and sheet size are determined, you can draw a border on-screen representing the maximum drawing area. The border reminds you that this is the maximum size diagram you can draw on that size sheet. All drawing objects must be contained within this border; otherwise they will fall outside the plotting area.Composing a Drawing LayoutCADD provides a number of special functions to compose a drawing layout. You can arrange diagrams on a sheet as you like and apply any scale factor. Different programs use different protocols to accomplish this task.

A common protocol used in AutoCAD and many other popular programs is as follows: To compose a drawing layout, you can activate a special mode of working called paper space or plot layout. The paper space represents the actual sheet size. You can arrange any number of diagrams on this sheet and apply a scale factor to diagrams individually. It shows exactly how the diagram will be plotted on the selected sheet size.

Another protocol used involves specifying the plotting scale and sheet size when you start a drawing. You enter all the drawing measurements in actual size and the computer automatically draws them to a selected scale. If you draw a 100’-0”x100’-0” square using a 1:100 scale, it will automatically scale it down and you can see exactly how it is going to fit on the sheet.

Regardless of which method is used for composing the drawing, you can create a border and title block that you would like to use for a project and store that drawing file in the CADD library. You can use this title block for all your drawings in the project for a consistent look.

The following table shows some of the standard sheet sizes (in inches):

US National CAD standards have established guidelines for sheet organization and the design of title blocks. Fig. 8.1 shows an example of a drawing created using these guidelines. The drawing area of the sheet is divided into columns and rows. Each column is identified with a number and each row is identified with a letter. This grid system helps to arrange the diagrams and to identify them using this grid system. A title block is located on the right side of the drawing. It contains blocks of information associated with the drawing and the project. National CAD Standards also include guideline to organize information within the title block. For more information about US National CAD standards, visit their web site at http://www.nationalcadstandard.org.

Selecting Text and Dimension Heights:

As diagrams are enlarged or reduced by applying a scale factor, the size of text, dimensions, patterns and symbols is also changed. When you place different scale diagrams on the same sheet, you may get different sized text for each diagram. This is generally not acceptable for professional drawings. It is better to have consistently sized text on the drawings regardless of their scale.

To achieve a standard text height for all the diagrams, you need to write text according to the scale of the diagrams. Let's say you want to print 1" high text for your final plots. If you like to use 1:50 scale for a diagram, you need to enter text 50 times larger than the printed size (50" high). When 1:50 scale factor is applied, it reduces the entire diagram (including text) by 1/50th. This results in 1" high printed text.

You can choose text heights that you would like to use in a project and prepare a table that shows the heights based on the scale of diagrams. Let’s say you want the printed text height to be 1/8”. You can calculate the text height for different scale diagrams using the following calculation:

There are hundreds of fonts available that you can use with CADD. The fonts used in the drawing must be compatible with the printer or plotter used for plotting.

Choosing Pens, Colors and Line weights:

CADD allows you to work with a variety of colors and line weights depending upon the plotter. In most CADD programs, the colors you use on-screen are configured with a specific line weight in the plotter. For example, the objects drawn with red color on-screen may be printed with .5mm line weight; the objects drawn with blue color may be printed with .2mm line weight. These are called pen assignments. You can choose any pen assignments for plotting.

Note: In most CADD programs line weights are not displayed on-screen, but the drawing is printed according to the line weights. Advanced programs include special functions, allowing you to view the line weights on-screen as well.

When setting up a plotter, one of the first steps is to configure line weights and colors available in the plotter with the CADD program. This means assigning each line weight in the plotter with a color or pen number in the drawing. You may want to make a table showing all the line weights and colors a plotter can produce and match them with the pen assignments you would like to use.

Most companies use their own pen assignments for plotting. If one company sends a drawing to other companies, it may not print the same way, because other companies may have their own pen assignments. US National CADD Standard has taken the initiative to solve this problem. They have established standards for pen assignments as described below:

Step Action1 Set up the plotter according to the manufacturer’s specifications and configure it with your CADD program.2 Place paper in the plotter and run a self test to ensure that the paper path is clear and the pens or cartridges are in good working condition.3 Display the drawing to be plotted on the screen and choose the Plot function.4 Respond to the specific prompts of your CADD program. In general, a CADD program will require the following information to plot the drawing:

Plotting area:You can plot a part of the drawing or the entire drawing. You will be able to indicate the plotting area by indicating a window (an imaginary rectangle formed by two diagonal points) or by selecting a specific view for plotting.

Plotting scale factor:Enter a scale factor based on how big or small you want to print the drawing and the sheet size used. (See topic “Selecting a Scale for Drawings.”)

Plotting origin:The plotting origin is a point that allows you to align the drawing shown on the screen with the paper in the plotter (see Fig. 8.2). You can place the diagrams on the paper anywhere by entering the exact coordinates of the plotting origin.

Important Tip:

It’s always better to do a test plot of a small area of the drawing before sending all the drawings to plotter or printer.

Printers and Plotters:

Printers and plotters are used to print CADD drawings. The drawings are generally printed at about 300-600 dpi (dots per inch) accuracy, which is considered quite high precision for engineering drawings. There are machines available that can print at 1200 dpi or higher accuracy as well.